Moderators: Dave Moll, Steve Anderson
Steve Anderson wrote:Indeed, there are many ways to consider how the 'S-curve' demodulator does its job, you could spend a long time analysing it and possibly end up being none the wiser. That is why I left the two filters as they were. All I did was improve(?) the full-wave rectification and change the rather strange original output filter.
The 'new' demod method. I doubt that it's new, I'm sure it's been done before though I've not seen any mention of it anywhere so far.
I've made a start on the hardware and possibly, with no interruptions, I might have something to put forward tomorrow.
Steve A.
Steve Anderson wrote:Hi Harry. The problem with using a PLL to demodulate the SSTV sub-carrier is that the modulation frequencies (DC to around 1kHz) and the sub-carrier frequencies (1200Hz-2300Hz, +/- side-bands) are so close together. This places an almost impossible task on the loop/low-pass filter. If the sub-carrier were (say) 10kHz-12kHz then it could be possible, though still not a cinch.
Although I had no interruptions yesterday I still ended up losing the best part of the day. My USB hub decided to quit on me and after a lot of mucking around I had to go out and buy a replacement. So not as much progress as I had hoped...a lot less!
I haven't forgotten the remaining circuits for your monitor that started this thread. I must make an effort to catch up!
Steve A.
Steve Anderson wrote:For the 5AHP7A there should be no need for an opto-isolator as apart from the nasty 7kV on the final anode, there's nothing on the CRT that semis can't handle directly. There's a good chance that one side of the heater and possibly the cathode could be earthed...or within a few dozen volts.
Steve A.
"Klaas Robers"]Harry, it is because of the magnetic deflection that the picture tube can be operated completely differently.
Using electrostatic deflection the deflection amplifiers should operate a a reasonable DC-voltage, say between +100 and +300 volt. That voltage is comparable to the final accelerator high voltage. So the cathode should be at a high negative voltage, and that is why the opto coupler is needed for the isolation.
Using magnetic deflection, the deflection coils are completely isolated from the HV circuit. Now the accellerator voltage may be +7 kV and all other electrodes of the picture tube can be at reasonable low voltages. To give an example:
- The cathode at +70 V,
- The grid at +30 V,
- The anode at +300V.
I did the video modulation in the cathode and the grid voltage is variable to control the brightness of black parts of the picture. The video modulator has a gain control (contrast) that keeps black (from the 1500 Hz input signal) at a fixed level. This is very handy, as you can control black and white with two separate potentiometers, which don't interact. From the FM-detector the video signal is DC coupled, there is no capacitor in the signal path.
Klaas Robers wrote:Harry, I doubt if the cathode would survive such an action. As far as I know they were activated after the tube was evacuated. The surface of cathodes was a metal that oxidizes very soon, so air in the tube is the definite end of the cathode.
The large tubes of the photo also had a deflection angle that was not so large, guess about 40 degrees.
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